The invention relates to electron discharge devices and particularly to photomultiplier tubes having a thermionic emission-reduction coating.
Photomultiplier tubes for use in severe environments, such as for oil-well logging, are described in U.S. Pat. No. 4,355,258, issued to G. N. Butterwick on Oct. 19, 1982, and incorporated by reference herein for the purpose of disclosure. Logging is a term given to the method of determining the mineral composition and structure of the geological media along bore holes.
Sensitive probes, or sondes, are used to determine the lithology, i.e., the character of the rock formation, including the density, of the media along the bore hole. The bore holes are typically thousands of meters deep and may exceed about ten-thousand meters. Temperature increases with bore hole depth, and the temperature in a ten-thousand meter deep hole may range between 100.degree. to 250.degree. C. In logging such a hostile environment, the sondes, which include a radioactive gamma ray source, such as cesium 137, and a detector comprising a sodium iodide crystal and a photomultiplier tube, are subjected to shock and vibration as well as to high operating temperatures.
Gamma rays from the cesium 137 source enter the medium surrounding the bore hole, and interactions occur among the gamma rays and the orbital electrons in the atoms of the material comprising the medium. The interactions impart energy to the orbital electrons and redirect or scatter photons of lower energy than the incident gamma rays in a direction different from that of the incident gamma rays. This effect is called the Compton Effect. Some of the scattered photons are detected by the sodium iodide crystal which converts them to luminous scintillations. The luminous scintillations are then detected by a photoemissive cathode and converted into electrical pulses by an electron multiplier of the photomultiplier tube. The electrical pulses represent Compton photon energy data which may then be converted into a geological formation-density log. A more complete description of oil-well logging is contained in an article by G. N. Butterwick, entitled, "Oil Exploration With Photomultiplier Tubes", published in the RCA Engineer, pp. 62-65 (Vol. 24, No. 5, February/March 1979).
The photoemissive cathode or photocathode of the photomultiplier tube is adversely affected by the high operating temperatures encountered in logging deep bore holes. As the temperature increases, the dark current of the tube, particularly the thermionic component of the dark current, also increases, thus decreasing the signal-to-noise ratio of the tube. Thermionic emission generally originates from the photocathode itself or from other surfaces in the tube on which alkali materials have been deposited, and is then amplified by the gain of the electron multiplier section of the tube. Typically, the photocathode is formed not only on the inside surface of the faceplate but also along the upper sidewall of the tube adjacent to the faceplate. FIG. 1 is a graph of the typical thermionic-emission current density for various types of photocathodes, as a function of temperature. Oil-well logging tubes, such as the RCA C31016G, utilize a high temperature, low-noise photocathode, such as the sodium-potassium-antimony (Na.sub.2 KSb) photocathode which is deposited in situ and is indicated at the extreme left-side of FIG. 1; nevertheless, at temperatures above 100.degree. C., the thermionic emission is severe. It is known in the art to cool the photomultiplier tube and reduce the thermionic emission by means of a cryostat; however, on some types of photocathodes, too cool a temperature may result in the photocathode becoming so resistive that the photoemission is blocked by a drop in potential across the photocathode surface. Another way of decreasing the thermionic emission is to minimize the electron emission surface of the photocathode by restricting the emission surface to the useful faceplate area and by preventing the formation of the photocathode on the sidewall. U.S. Pat. No. 3,372,967, issued to F. R. Hughes on March 12, 1968, discloses an antimony evaporator shield which restricts the deposition of antimony to the faceplate of the tube. The subsequently deposited alkali metals react with the antimony to form a photocathode only on the faceplate of the tube. Such a shielding structure is not always feasible, especially in a small tube such as the C31016G, where the mechanical shield may interfere with the electron optics of the tube, and other means are frequently required to restrict the cathode area so as to minimize thermionic emission.
U.S. Pat. No. 3,327,152, issued to A. L. Greilich on June 20, 1967, discloses photoemissive retardant agents, such as iron, tin, lead and the chloride of nickel, which are deposited on a grid of a grid-controlled phototube and interact with the alkali metals to provide a high work function grid surface from which the electrons cannot escape, thus reducing the dark current emission from the grid. The photoemissive cathode described in the Greilich patent utilizes a metal substrate for a structural support. Antimony is deposited on the substrate prior to the mounting of the substrate, grid and anode in the tube envelope. An alkali material, such as cesium, is introduced into the tube to react with the antimony and to form the photoemissive cathode. The retardant agents described in the Greilich patent would be ineffective in reducing thermionic emission from Applicants' photomultiplier tube, since Applicants' photocathode, including the base layer of antimony, is deposited in situ. In such a structure, antimony would cover the retardant agents, and a photoemissive cathode would be formed over the retardant material, and therefore no decrease in thermionic emission would occur.